A common method of manufacturing an optical recording medium for recording, reproducing, erasing etc. on both sides of the recording medium comprises, as shown in FIG. 4(a), the steps of first forming recesses and/or protrusions 2 . . . such as grooves and pits on one side of a transparent substrate 1 and then integrally forming a recording film 3 to cover these recesses and/or protrusions, as shown in FIG. 4(b) to thus form an optical recording substrate 4. Thereafter, as shown in FIG. 4(c), an optical recording substrate 4' is formed in a like way and with their recording films 3 and 3' opposed to each other, the optical recording substrates 4 and 4' are bonded to each other by the use of an adhesive 5, for instance, an ultraviolet ray hardening resin.
The aforementioned substrates 1 and 1' are, however, required to allow transmission of light for recording, reproducing and erasing information and since the recesses/protrusions 2 and 2' . . . have to be formed, there are various conditions or requirements for such substrates 1 and 1'.
For instance, they are required to be (1) transparent; (2) optically isotropic, and in particular, have a small birefringence; (3) flat with little indication of warping or waviness; and (4) easy to form the recesses and/or protrusions. Furthermore, there are various requirements for the substrates 1 and 1' also relating to the recording films 3 and 3': for instance, they are required to have a high resistance to heat if the method of forming the recording films 3 and 3' requires heating, and if the films 3 and 3' are required to be moisture-proof, they are required to have a low moisture permeability and low water absorption.
Recently, therefore, study has been made of the possibility of using as the aforementioned substrates 1 and 1' those of polycarbonate resin made by injection molding (hereinafter called "PC substrate").
Even the aforementioned PC substrate made by injection molding tends to have a high birefringence, and when the recording film 3 is formed thereon to make the optical recording substrate 4, the substrate tends to form a convex warp on the side covered with the recording film 3, as shown in FIG. 5. This warping phenomenon in the optical recording substrate 4 warping on the side covered with the recording film 3 is due to the difference in thermal expansion coefficient between the recording film 3 and the PC substrate 1, and if the temperature is raised at the time of forming the recording film 3, the dimensions of the PC substrate 1 becomes smaller than that of the recording film 3.
Also, when an optical recording medium is made by bonding together the optical recording substrates 4 and 4', such an optical recording medium is constantly subject to some degree of stress. When this optical recording medium is subjected to sudden changes in temperature, exfoliation of the recording film 3 from the substrate 1, or of each substrate 1 and 1' takes place in the optical recording medium, thereby affecting the reliability of the optical recording medium.